Dec 24, 2022

Where have nitrogen and deuterium gone in the Universe?

Near-infrared spectroscopy of a young star reveals the production process of nitrogen-containing molecules and localizes missing deuterium in interstellar space.


December 24, 2022

Using the Infrared Camera on board the AKARI satellite, Japanese researchers provide the first clear evidence that ultraviolet radiation helps produce nitrogen-containing molecules and that organic molecules harbor missing deuterium in interstellar space. The discovery has implications for our understanding of how prebiotic organic matter, such as amino acids, may have originated in interstellar space, and how matter evolved in the Universe. A team of researchers from the University of Tokyo, Meisei University, and Niigata University made the discoveries. They published the results in the latest issue of The Astrophysical Journal.

Deuterium is an isotope of Hydrogen, which contains one proton and one neutron, and is also known as heavy hydrogen. All the Universe’s deuterium formed in the first few moments after the Big Bang. But it perishes gradually and turns into other nuclei in a process called nucleosynthesis. More than half of the deuterium occurs in a gaseous form and some amount of deuterium bonds with hydrogen or organic matter. But observations so far have failed to confirm that deuterium may be hiding with organic matter in the interstellar medium. Understanding where the Universe’s deuterium has gone can help scientists reveal the evolution of matter in the Universe. Similarly, chemical processes involving nitrogen in low-temperature environments that led to the formation of prebiotic organic matter, such as amino acids, were unclear.

“We made use of the observational data taken with the Japanese infrared satellite AKARI of a high-mass young star, whose envelope provides special physical conditions of a combination of strong ultraviolet radiation and low temperature, both of which are crucial to the present findings. Objects with such physical conditions have not fully been explored in past studies,” said Takashi Onaka, Professor Emeritus at The University of Tokyo, and the first author of the research article.

Figure 1 : Pseudo color image of the stellar object AFGL 2006. The light blue rectangle shows the region, where the scientists sampled the spectra. The image was constructed from the data taken from the NASA/IPAC data archive (


A prism can help us see the spectral components of visible light. Similarly, infrared spectroscopy allowed scientists to probe the components of the absorption and emission spectra from the young star in the near-infrared wavelength range (from 2.5 to 5 µm). In interstellar space, various gases mingle with dust which can absorb or scatter radiation, making it difficult to precisely identify them. But infrared spectroscopy can provide information about the mass of different matter based on their spectral signature.

“We are interested in the chemistry of the formation of ice species [such as H2O, CO2, or OCN- (cyanate) ices] in the interstellar space as well as physics of organic matter, particularly of polycyclic aromatic hydrocarbons (PAHs) in the Universe, which are believed to be present in the interstellar space and play roles in the energy balance and physics of the interstellar matter.  These may be directly related to the formation of prebiotic matter in the Universe,” added Prof. Onaka.

The infrared spectra from the young star revealed absorption features of H2O and CO2 ices at 3 µm and 4.26 μm wavelengths. They also showed a broad, complex absorption feature at around 4.65 μm which the scientists attributed to the nitrogen-containing molecule OCN-, CO ice, and CO gas. The spectral features suggested that ultraviolet light emanating from the star’s center enhances the formation of nitrogen-containing molecules. The team also attributed an excess emission at around 4.4 µm to Carbon Deuterium aromatic bond (C—D) vibration, which has been elusive in earlier studies. The result shows that deuterium hides in organic matter. The scientists attributed another emission feature at 3.3 µm to PAHs). The PAHs contain interstellar carbon, and they can store deuterium. So, they concluded that low temperature and ultraviolet radiation on the young star help produce nitrogen-containing molecules and allow missing deuterium to bond with carbon. 

The study used low-resolution spectroscopy to probe the young star in the Galactic center. Due to the low-resolution, the spectral components of the 4.56 µm feature could not be fully resolved.

Follow-up observations with the James Webb Space Telescope now in orbit can extend this study, leading to a full understanding of the initial process of amino-acid formation or prebiotic matter in the interstellar space, as well as the identification of the interstellar phase that harbors the missing deuterium, Prof. Onaka commented. 

Figure 2 : A schematic view of the observed region. The central star AFGL 2006 emits ultraviolet radiation, ionizing the gas surrounding it. Around the ionized gas, the neutral gas region spreads, and the layer that contains the ice species including OCN- is situated between the neutral gas and us, the observers. The excess emission at 4.4 μm (the spectral trace shown above the observer) is emitted from the boundary between the neutral gas and the ice layer. 


Publication details

 Journal The Astrophysical Journal
Near-infrared spectroscopy of a massive young stellar object in the direction toward the Galactic Center: XCN and aromatic C—D features
Takashi Onaka, Itsuki Sakon, Takashi Shimonishi


The authors of the paper would like to acknowledge that this work is based on the observations of AKARI, a JAXA project with the participation of ESA.


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